P Ehses

Measuring RF-induced currents inside implants: Impact of device configuration on MRI safety of cardiac pacemaker leads

Radiofrequency (RF)‐related heating of cardiac pacemaker leads is a serious concern in magnetic resonance imaging (MRI). Recent investigations suggest such heating to be strongly dependent on an implants position within the surrounding medium, but this issue is currently poorly understood. In this study, phantom measurements of the RF‐induced electric currents inside a pacemaker lead were performed to investigate the impact of the device position and lead configuration on the amount of MRI‐related heating at the lead tip. Seven hundred twenty device position/lead path configurations were investigated. The results show that certain configurations are associated with a highly increased risk to develop MRI‐induced heating, whereas various configurations do not show any significant heating. It was possible to precisely infer implant heating on the basis of current intensity values measured inside a pacemaker lead. Device position and lead configuration relative to the surrounding medium are crucial to the amount of RF‐induced heating in MRI. This indicates that a considerable number of implanted devices may incidentally not develop severe heating in MRI because of their specific configuration in the body. Small variations in configuration can, however, strongly increase the risk for such heating effects, meaning that hazardous situations might appear during MRI. Magn Reson Med, 2009.

Improved Radial GRAPPA Calibration for Real-Time Free-Breathing Cardiac Imaging

To generate real‐time, nongated, free‐breathing cardiac images, the undersampled radial trajectory combined with parallel imaging in the form of radial GRAPPA has shown promise. However, this method starts to fail at high undersampling factors due to the assumptions that must be made for the purposes of calibrating the GRAPPA weight sets. In this manuscript, a novel through‐time radial GRAPPA calibration scheme is proposed which greatly improves image quality for the high acceleration factors required for real‐time cardiac imaging. This through‐time calibration method offers better image quality than standard radial GRAPPA, but it requires many additional calibration frames to be acquired. By combining the through‐time calibration method proposed here with the standard through‐k‐space radial GRAPPA calibration method, images with high acceleration factors can be reconstructed using few calibration frames. Both the through‐time and the hybrid through‐time/through‐k‐space methods are investigated to determine the most advantageous calibration parameters for an R = 6 in vivo short‐axis cardiac image. Once the calibration parameters have been established, they are then used to reconstruct several in vivo real‐time, free‐breathing cardiac datasets with temporal resolutions better than 45 msec, including one with a temporal resolution of 35 msec and an in‐plane resolution of 1.56 mm2. Magn Reson Med, 2011.

Spatial distribution of RF-induced E-fields and implant heating in MRI

The purpose of this study was to assess the distribution of RF‐induced E‐fields inside a gel‐filled phantom of the human head and torso and compare the results with the RF‐induced temperature rise at the tip of a straight conductive implant, specifically examining the dependence of the temperature rise on the position of the implant inside the gel. MRI experiments were performed in two different 1.5T MR systems of the same manufacturer. E‐field distribution inside the liquid was assessed using a custom measurement system. The temperature rise at the implant tip was measured in various implant positions and orientations using fluoroptic thermometry. The results show that local E‐field strength in the direction of the implant is a critical factor in RF‐related tissue heating. The actual E‐field distribution, which is dependent on phantom/body properties and the MR‐system employed, must be considered when assessing the effects of RF power deposition in implant safety investigations. Magn Reson Med 60:312–319, 2008.

IR TrueFISP with a golden-ratio-based radial readout: Fast quantification of T1, T2, and proton density

A promising approach for the simultaneous quantification of relative proton density (M0), T1, and T2 is the inversion‐recovery TrueFISP sequence, consisting of an inversion pulse followed by a series of balanced steady‐state free precession acquisitions. Parameters can then be obtained from a mono‐exponential fit to the series of images. However, a segmented acquisition is usually necessary, which increases the total acquisition time considerably. The goal of this study is to obtain M0, T1, and T2 maps using a single‐shot acquisition, with T1 and T2 measurements in brain that are consistent with the published literature, with a 20‐fold speed improvement over the segmented approach, and at a clinically relevant spatial resolution. To this end, a single‐shot inversion‐recovery TrueFISP sequence was combined with a radial view‐sharing technique. The parameters M0, T1, and T2 were then obtained on a pixel‐wise basis from a three fit parameter to the signal evolution. The accuracy of this method for quantifying these parameters is demonstrated in vivo. In addition, further corrections to the quantification necessary owing to other experimental factors, namely magnetization transfer and imperfect slice profiles, were developed. Including additional scans necessary for these corrections in the measurement protocol, the required scan time is increased from approximately 6 to 18‐28 s per slice. Magn Reson Med, 2013.

Improved temporal resolution in cardiac imaging using through-time spiral GRAPPA

Previous work has shown that the use of radial GRAPPA for the reconstruction of undersampled real‐time free‐breathing cardiac data allows for frame rates of up to 30 images/s. It is well known that the spiral trajectory offers a higher scan efficiency compared to radial trajectories. For this reason, we have developed a novel through‐time spiral GRAPPA method and demonstrate its application to real‐time cardiac imaging. By moving from the radial trajectory to the spiral trajectory, the temporal resolution can be further improved at lower acceleration factors compared to radial GRAPPA. In addition, the image quality is improved compared to those generated using the radial trajectory due to the lower acceleration factor. Here, we show that 2D frame rates of up to 56 images/s can be achieved using this parallel imaging method with the spiral trajectory. Magn Reson Med, 2011.

MRI thermometry: Fast mapping of RF‐induced heating along conductive wires

Conductive implants are in most cases a strict contraindication for MRI examinations, as RF pulses applied during the MRI measurement can lead to severe heating of the surrounding tissue. Understanding and mapping of these heating effects is therefore crucial for determining the circumstances under which patient examinations are safe. The use of fluoroptic probes is the standard procedure for monitoring these heating effects. However, the observed temperature increase is highly dependent on the positioning of such a probe, as it can only determine the temperature locally. Temperature mapping with MRI after RF heating can be used, but cooling effects during imaging lead to a significant underestimation of the heating effect. In this work, an MRI thermometry method was combined with an MRI heating sequence, allowing for temperature mapping during RF heating. This technique may provide new opportunities for implant safety investigations. Magn Reson Med, 2008.

Using the GRAPPA operator and the generalized sampling theorem to reconstruct undersampled non-Cartesian data

As expected from the generalized sampling theorem of Papoulis, the use of a bunched sampling acquisition scheme in conjunction with a conjugate gradient (CG) reconstruction algorithm can decrease scan time by reducing the number of phase‐encoding lines needed to generate an unaliased image at a given resolution. However, the acquisition of such bunched data requires both modified pulse sequences and high gradient performance. A novel method of generating the “bunched” data using self‐calibrating GRAPPA operator gridding (GROG), a parallel imaging method that shifts data points by small distances in k‐space (with Δk usually less than 1.0, depending on the receiver coil) using the GRAPPA operator, is presented here. With the CG reconstruction method, these additional “bunched” points can then be used to reconstruct an image with reduced artifacts from undersampled data. This method is referred to as GROG‐facilitated bunched phase encoding (BPE), or GROG‐BPE. To better understand how the patterns of bunched points, maximal blip size, and number of bunched points affect the reconstruction quality, a number of simulations were performed using the GROG‐BPE approach. Finally, to demonstrate that this method can be combined with a variety of trajectories, examples of images with reduced artifacts reconstructed from undersampled in vivo radial, spiral, and rosette data are shown. Magn Reson Med, 2009.

Dynamically phase‐cycled radial balanced SSFP imaging for efficient banding removal

Balanced steady‐state free precession (bSSFP) imaging suffers from banding artifacts due to its inherent sensitivity to inhomogeneities in the main magnetic field. These artifacts can be removed by the acquisition of multiple images at different frequency offsets. However, conventional phase‐cycling is hindered by a long scan time. The purpose of this work is to present a novel approach for efficient banding removal in bSSFP imaging.

DC-gated high resolution three-dimensional lung imaging during free-breathing

To use the acquisition of the k‐space center signal (DC signal) implemented into a Cartesian three‐dimensional (3D) FLASH sequence for retrospective respiratory self‐gating and, thus, for the examination of the whole human lung in high spatial resolution during free breathing.

Quantitative and functional pulsed arterial spin labeling in the human brain at 9.4 t

The feasibility of multislice pulsed arterial spin labeling (PASL) of the human brain at 9.4 T was investigated. To demonstrate the potential of arterial spin labeling (ASL) at this field strength, quantitative, functional, and high‐resolution (1.05 × 1.05 × 2 mm3) ASL experiments were performed.